In the heaviest atoms there is often a large inner tension. They may thereby emit radiation into their surroundings. There are three different forms of radiation: alpha, beta and gamma radiation. Gamma radiation is pure electromagnetic radiation. It can penetrate most materials in the same way as X-rays. Alpha and beta radiation are caused by the atom breaking down, reducing the size of the nucleus. Radium (Ra) with the atomic number 88, will decay by radiation through a number of stages and finally become lead (Pb) with the atomic number 82. This process takes thousands of years.
The periodic table.
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